The present invention relates to a piezoelectric component for obtaining a filter comprising series and parallel resonators which are connected to each other in the form of a ladder, and a method of manufacturing thereof.
A ladder type filter is widely applied to an IF filter for a portable radio communication device or a taxi radio communication device. FIGS. 1 and 2 show an example of a conventional four-element (two-section) ladder type filter. This filter comprises a box case 1 which stores two series ceramic resonators 2 and 3, two parallel ceramic resonators 4 and 5, an input terminal plate 6, a ground terminal plate 7, an output terminal plate 8, a connecting terminal plate 9 and a spring plate 10. FIG. 3 shows its electrical connection.
The input terminal plate 6 is provided on its one surface with a projection 6a which comes into pressure contact with a central portion of the series resonator 2. The ground terminal plate 7 is provided on its both surfaces with projections 7a and 7b which come into pressure contact with central portions of the two parallel resonators 4 and 5. The output terminal plate 8 is provided on its both surfaces with projections 8a and 8b which come into pressure contact with central portions of the parallel resonator 5 and the series resonator 3, respectively. The connecting terminal plate 9 has a portion 9a which is inserted between the series resonator 2 and the parallel resonator 4 and another portion 9b which is arranged outside of the series resonator 3. These portions 9a and 9b are coupled with each other by a coupling portion 9c. The spring plate 10 is arranged between the portion 9b and an interior surface of the case 1, to bring the terminal plates 6-9 into pressure contact with the resonators 2-5, thereby electrically connecting all of the parts 2-9.
After the resonators 2-5, the terminal plates 6-9 and the spring plate 10 are inserted into the case 1 in the aforementioned manner, a stopper sheet 11 having slits is set into an opening of the case 1. Then a cavity defined by the sheet 11 and the opening is filled up with resin 12 so that the case 1 is sealed. Respective lead portions 6b, 7c and 8c of the terminal plates 6, 7 and 8 are projected from the case 1.
When the aforementioned parts are assembled into the filter, the resonators 2-5 are brought into contact with the terminal plates 6-9 in such instable states that it is impossible to simultaneously insert a plurality of such resonators or terminal plates into the case 1. In general, therefore, such parts are manually inserted one by one into the case 1. Consequently, a long time is required for assembling those parts and working efficiency is lowered. When the aforementioned filter is automatically assembled by a robot or the like, the operating speed thereof cannot be increased since it is necessary to align the parts one by one along the thickness direction. Further, it is difficult to accurately locate the respective parts, and hence the conventional filter is unsuitable for automatic assembling.
Further, a ladder type filter with four or more elements increases the manufacturing cost since parts having complicated shapes such as the connecting terminal plate 9 are required. Moreover, a difficult operation is required for simultaneously inserting such parts into a plurality of gaps between other parts.
In addition, in a resin-molded ladder type filter, it is necessary to set a stopper sheet in an opening of a case in order to prevent resin from flowing into the case. This also may reduce working efficiency.